Draft

Advisory Board on Radiation and Worker Health

National Institute for Occupational Safety and Health

SC&A Commentary on NIOSH’s Approach to Quantifying 

Outdoor and Indoor Airborne Dust Loadings

Contract No. 75D30119C04183

Document No. SCA-TR-2021-SEC005, Revision 0

Prepared by

Rose Gogliotti, MS

John Mauro, PhD, CHP

Lynn Anspaugh, PhD

SC&A, Inc.

2200 Wilson Blvd., Suite 300

Arlington, VA 22201-3324

October 25, 2021

DISCLAIMER

This is a working document provided by the Centers for Disease Control and Prevention (CDC) 

technical support contractor, SC&A for use in discussions with the National Institute for 

Occupational Safety and Health (NIOSH) and the Advisory Board on Radiation and Worker 

Health (ABRWH), including its Working Groups or Subcommittees. Documents produced by 

SC&A, such as memorandum, white paper, draft or working documents are not final NIOSH or 

ABRWH products or positions, unless specifically marked as such. This document prepared by 

SC&A represents its preliminary evaluation on technical issues.

NOTICE: This document has been reviewed to identify and redact any information that is 

protected by the Privacy Act 5 U.S.C. § 552a and has been cleared for distribution. 
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SC&A, Inc. technical support for the Advisory Board on Radiation and Worker Health’s 

review of NIOSH dose reconstruction program

Document title SC&A Commentary on NIOSH’s Approach to Quantifying Outdoor and 

Indoor Airborne Dust Loadings

Document number SCA-TR-2021-SEC005

Revision number 0 (Draft)

Supersedes NA

Effective date October 25, 2021

Task manager Rose Gogliotti, MS [signature on file]

Project manager Bob Barton, CHP [signature on file]

Document reviewer(s) Bob Barton, CHP [signature on file]

Record of revisions

Revision number Effective date Description of revision

0 (Draft) 10/25/2021 Initial issue
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Table of Contents

List of Tables................................................................................................................... 4

List of Figures.................................................................................................................. 4

Abbreviations and Acronyms........................................................................................... 5

1 Introduction............................................................................................................... 6

2 Background .............................................................................................................. 6

3 Dust Loading Data.................................................................................................... 7

4 Surrogate Data Issues ............................................................................................ 11

5 Literature Review.................................................................................................... 12

5.1 Outdoor dust loadings ...................................................................................... 12

5.2 Indoor dust loadings......................................................................................... 17

6 Enrichment/Enhancement ...................................................................................... 18

7 Conclusions ............................................................................................................ 20

8 References ............................................................................................................. 21
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List of Tables

Table 1. Summary statistics of dust loading.................................................................... 9

Table 2. 98th percentile values (PM 2.5) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020........................................................ 14

Table 3. Weighted average values (PM 2.5) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020........................................................ 14

Table 4. Second maximum values (PM 10) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020........................................................ 14

Table 5. Mean 24-hour values (PM 10) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020........................................................ 15

List of Figures

Figure 1. Histograms faceted by monitoring area............................................................ 8

Figure 2. Analysis of possible distributions.................................................................... 10

Figure 3. Plot of enhancement factors measured over undisturbed and disturbed soils20
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Abbreviations and Acronyms

ABRWH, Board Advisory Board on Radiation and Worker Health 

DOE U.S. Department of Energy 

EPA U.S. Environmental Protection Agency 

g/m3 gram per cubic meter

hr hour 

M&C Metals and Controls Corporation 

mg/m3 milligram per cubic meter

μg/m3 microgram per cubic meter

μm micrometer

NAPCA National Air Pollution Control Administration 

NIOSH National Institute for Occupational Safety and Health

NRC U.S. Nuclear Regulatory Commission

ORAUT Oak Ridge Associated Universities Team

pCi/g picocurie per gram 

pCi/m2 picocurie per square meter

pCi/m3 picocurie per cubic meter 

PM 10 particulate matter <10 μm 

PM 2.5 particulate matter <2.5 μm 

SEC Special Exposure Cohort 

SME subject-matter expert

SRDB Site Research Database

TBD technical basis document

WG Work Group 
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1 Introduction

Adequately characterizing the amount of airborne radioactive dust available for inhalation and 

ingestion is an integral factor in modeling potential exposures based on contamination survey 

data. In its January 21, 2021, response paper, the National Institute for Occupational Safety and 

Health (NIOSH, 2021, p. 12) discusses NIOSH’s approach for deriving a “Dust Load Factor for 

Inside and Outside Subsurface Work” for use in support of the Metals and Controls Corp. 

(M&C) Special Exposure Cohort (SEC) investigations underway by the Advisory Board on 

Radiation and Worker Health (Board). That section states the following: 

NIOSH examined an excavation at the Mound site and determined it to be a 

useful general model for dust loading during excavations of soils and plans to 

include it in the next ORAUT-OTIB-0070 revision [[NIOSH,] 2012]. This model 

is directly applicable to M&C’s Outside Area excavations and conservatively 

bounds M&C’s Inside Building 10 work. 

This statement indicates that NIOSH plans to use 212 micrograms per cubic meter (μg/m3

) 

(2.12E-4 grams per cubic meter (g/m3

)) as a default value for outdoor and indoor dust loads 

associated with excavations at M&C. NIOSH further indicates that this general model will be 

included in a future revision of ORAUT-OTIB-0070 (“OTIB-0070”). During the March 18, 

2021, M&C Work Group (WG) meeting, the WG asked SC&A to evaluate the 

representativeness of NIOSH’s dust loading values for dust loadings from M&C indoor and 

outdoor excavations (M&C WG, 2021a). Additionally, the WG asked SC&A to evaluate the 

appropriateness of using the data as a generic default value for incorporation into OTIB-0070. 

2 Background

NIOSH originally proposed a dust loading value of 220 μg/m3 in its October 24, 2018, white 

paper, “Metals and Controls Corp. Maintenance Worker Exposure Model.” In this paper, NIOSH 

compared the use of the Mound site air monitoring data against the surrogate data criteria from 

OCAS-IG-004. SC&A notes that the criteria listed in OCAS-IG-004, revision 0 (NIOSH, 2008), 

are not the same surrogate data criteria that the Board adheres to. The use of 220 μg/m3 was 

supported by email correspondence cited by NIOSH (2018) that documented a study conducted 

for the Mound Plant Canal Cleanup Project (hereafter referred to as “the Mound data” or “the 

Mound study”). 

During the September 2, 2020, M&C WG meeting, Board members indicated that there was not 

enough information available to interpret the Mound data (M&C WG, 2020). NIOSH 

volunteered to interview the subject-matter expert (SME) on the study to provide additional 

information on the air sampling work that was completed at Mound. This interview was 

conducted on October 15, 2020, with only representatives from NIOSH present. The purpose of 

this interview was to collect and document the SME’s experience in measuring the airborne dust 

loadings that occurred outdoor at the Mound facility while excavating soil and sediment in a 

contaminated drainage canal at Mound. The original interview is documented in NIOSH (2020) 

and includes data, photographs, and discussions between the SME and NIOSH representatives. 
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On January 21, 2021, NIOSH issued a response paper that modified the original 2018 dust 

loading study. The paper discussed additional data that modified the original dust loading 

calculations (NIOSH, 2021, p. 12): 

There were three areas monitored at Mound: the excavation itself, the staging 

area, and the support area. The excavation area had the highest concentrations at 

213 μg/m3, followed by the staging area (where a front-end loader dumped soils 

into railroad cars) at 212 μg/m3

, and the support area, which had the lowest 

concentrations at 137 μg/m3. NIOSH did not use the data from the support area 

because it was described as a non-working, background type of area. Using this 

more complete data set, NIOSH calculated an empirical 95th percentile value of 

212 μg/m3 and can use this value in conjunction with the 95th percentile uranium 

and thorium concentrations to bound exposures during subsurface work. 

At the subsequent April 2021 WG meeting, NIOSH presented the results of this interview and 

the January 21, 2021, white paper to the WG. During the meeting, the WG judged that it would 

be prudent to repeat and perhaps expand upon the interview with the active participation of the 

M&C WG and representatives of SC&A (M&C WG, 2021b). The WG believed that additional 

information might be helpful in understanding the applicability of these airborne dust-loading 

measurements to modeling both outdoor and indoor dust loadings during excavations at M&C 

during the residual period. An additional consideration was the possible use of the Mound data as 

the basis for developing generic models that can be incorporated into OTIB-0070 for use at other 

sites.

On May 20, 2021, SC&A representatives participated in a re-interview of the SME conducted by 

representatives of NIOSH and the M&C WG. The re-interview followed the same structure as 

the original interview; however, Board members and SC&A representatives were given an 

opportunity to raise questions to help gain a better understanding of the Mound excavation and 

the data obtained from it. During that interview, SC&A requested the digitized data obtained 

from the Mound study; NIOSH provided this information to SC&A as an Excel file on 

May 26, 2021. 

3 Dust Loading Data

In the January 21, 2021, response paper, NIOSH indicated that its analysis resulted in the 

following dust loadings from each area:

• Excavation area: 213 μg/m3

• Support area: 212 μg/m3

• Staging area: 137 μg/m3

However, the paper is silent as to what these values actually represent. SC&A reviewed the data 

and found there were 338 lines of data available in the dataset that contained relevant 

information on each sampling event, including start and stop time/date stamps, sampling location 

information, and initial and final filter weights. SC&A was able to verify that the values 

presented in the NIOSH response represented the empirical 95th percentile values of each area. 
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Without omitting negative values, SC&A was able to replicate the combined support and 

excavation empirical 95th percentile value of 212 μg/m3 presented in the response paper.

When independently analyzing the data, SC&A identified one row with no masses reported; this 

data point was omitted because of the missing data. Additionally, 13 rows from the staging area 

were identified where the final mass was less than the initial mass. SC&A assumed there was an 

error with the data collection or reporting of these samples and omitted them from the analysis. 

This left 324 samples for analysis. 

SC&A plotted the data as faceted histograms by sampling area in figure 1. From the histograms, 

it is readily apparent the support area in general had a lower dust loading than the excavation and 

staging areas. This is further supported by the summary statistics SC&A generated on the Mound 

dust loading samples in table 1. The support area was described by NIOSH as a “background” 

area where no excavation or dust creating actives occurred. SC&A agrees it is reasonable to 

remove these data from the dust-loading assessment.

Figure 1. Histograms faceted by monitoring area
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Table 1. Summary statistics of dust loading

Location of 

samples

No. 

samples 

included 

No. samples 

omitted

Minimum 

(μg/m3) 

Maximum 

(μg/m3) 

Mean 

(μg/m3) 

Median 

(μg/m3) 

Excavation area 97 1 0.38 327.93 80.81 59.50

Staging area 204 13 3.42 893.23 96.35 78.42

Support area 23 0 19.81 157.40 60.57 46.18

All locations 324 14 0.38 893.23 89.16 71.00

Staging and 

excavation areas 301 14 0.38 893.23 91.34 72.68

Further looking at the data, SC&A noted a single data point in the staging area that was 

significantly larger than all other dust loading values calculated: 893.23 μg/m3. SC&A looked at 

the other dust loadings obtained from the same day and same location and found that the next 

closest dust loading value was approximately 6 times smaller than this sample. This is 

unexpected because samplers were placed around the dust-generating activities and were 

sampling simultaneously; however, since it is possible that an excavation could produce an 

extreme value, the sample was left in the data. 

SC&A additionally assessed the fit of the data. From figure 1, Weibull, lognormal and gamma 

distributions were identified as possible distributions based on the shape of the data. In figure 2, 

SC&A plotted the data against theoretical distributions. While all three distributions in figure 2 

visually have reasonable fits to the data distribution, from the QQ plot in figure 2, it can be 

inferred that the lognormal deviates from the theoretical tail at the upper end of the distribution.
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Figure 2. Analysis of possible distributions

Because differences in fit were not visibly apparent in figure 2, SC&A tabulated various 

goodness-of-fit statistics and criteria (Kolmogorov-Smirnov statistic, Cramer-von Mises statistic, 

Anderson-Darling statistic, Akaike's information criterion, Bayesian information criterion) for 

both Weibull and gamma distributions. For all goodness-of-fit values tested, the gamma 

distribution was a stronger fit to the Mound dust loading data.

SC&A calculated the 95th percentile of a gamma distribution from the Mound data to be 

222 μg/m3

. This is modestly larger though reasonably similar to the empirical 95th percentile 

value of 212 μg/m3 that NIOSH calculated. SC&A notes that this dust loading when applied at 

M&C makes no adjustment for background (ambient) levels of dust in the air, which adds an 

additional degree of conservatism to the calculation.
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4 Surrogate Data Issues

The Mound data and the data for indoor and outdoor excavation dust loadings described in 

section 3 can be considered a type of surrogate data, the use of which is governed by the Board’s 

surrogate data criteria(ABRWH, 2010).

1 The Board’s surrogate data criteria were developed 

primarily for radiological data characterizing external and internal doses and a facility’s 

radiological characteristics. In this case, the following Board criteria apply:

1 The Board’s surrogate criteria are somewhat different than the surrogate criteria in OCAS-IG-004 (NIOSH, 

2008), which NIOSH (2018) used to assess the applicability of the data to M&C.

1. Hierarchy of Data: The hierarchy of data stipulates the preference that should be placed 

on various types of data. There are no covered individual bioassays or air monitoring 

from M&C during the residual period; therefore, there are no data that should take 

precedence over the Mound data.

2. Exclusivity Requirements: SC&A is aware of limited air sampling that occurred during 

remediation (1992 and July 1995–September 1996). These air sampling results were 

deemed not applicable to the M&C workforce by the M&C WG and petitioners because 

the work done while sampling was done with awareness of radiological hazards. Since 

there are no applicable air monitoring data from the site, surrogate data must be used to 

establish dust loading. However, site-specific data are still being used to quantify the 

radiological contamination on site.

3. Site and Process Similarities: The surrogate data criteria regarding outdoor and indoor 

dust loading during excavation activities certainly apply to these same activities as those 

at M&C and can also be considered generically applicable to outdoor and indoor 

excavation activities. However, for both indoor and outdoor excavation activities, there 

are likely many site-specific characteristics that can uniquely affect dust loadings, 

including the characteristics of the soil and the proximity of workers to the excavation 

activities. There is very little that can be done to accommodate these types of site-specific 

characteristics, except to use a degree of professional judgement that would tend to place 

an upper bound on the dust loading. 

For example, as applied to M&C, many interviews indicated that there was a high water 

table on site and that rains would cause backups and flooding. One worker indicated that 

when they dug in Building 4 (next to Building 10), they used sump pumps to remove 

water (NIOSH, 2017a, p. 3). Another worker recalled that when they would dig outdoors, 

they would hit water at around 6 to 8 feet deep (Worker 8, p. 9). When it rained heavily, 

the areas around Building 10 flooded due to poor drainage (NIOSH, 2017b, p. 9), and 

sometimes the water coming off the roof would back up (NIOSH, 2017b, p. 4). A worker 

recalled that when water would back up from the roof, “Some of the water would have to 

come out on the floor” (NIOSH, 2017a, p. 3). 

Digging in moist soil generates less dust than dry soil. SC&A believes that the presence 

of water likely results in an overestimate of the average dust loading over the time 
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periods during which outdoor and indoor repurposing and maintenance activities were 

performed at M&C, as compared to the Mound data and the literature review data 

summarized in this report. However, this cannot be said for the use of the 212 μg/m3 for 

use as a generic value for outdoor and indoor excavations at some unknown facility or 

site. Hence, its use as a generic dust loading in OTIB-0070 should be uniquely evaluated 

at each site of proposed use.

4. Temporal Considerations: The Mound excavations occurred in 1996, while M&C 

excavations occurred from 1968 through 1996. The passage of time has not significantly 

altered backhoe operations, hand digging, or their ability to generate airborne dust. 

SC&A notes that the Mound study did have only minimal hand shoveling, while the 

M&C excavation work, especially indoors, had more significant hand shoveling. 

5. Plausibility: SC&A deems the dust loading results of the Mound study to be 

scientifically plausible. This determination was made by analysis of the data obtained 

from the Mound study, which had a range consistent with values expected from 

excavation activities.

5 Literature Review

The term “dust load factor” (or “dust loading”) is defined as the short-term or long-term average 

mass of airborne dry dust of respirable size particles per cubic meter of air. As used in the 

context of outdoor resuspension of soil (as opposed to outdoor airborne dust resulting from the 

release of emissions to the atmosphere from commercial and industrial operations), airborne dust 

is produced by disturbing the soil, such as by wind erosion, walking on the soil, vehicular traffic, 

soil-excavation activities, etc. In addition, the degree to which these disturbances create airborne 

dust is very much dependent on the characteristics of the soil; e.g., wet vs. dry soil, clay vs. loam 

vs. sand, vegetative cover, etc. (Anspaugh et al., 1975). SC&A identified several publications 

that address outdoor and indoor dust loadings, as summarized in sections 5.1 and 5.2. 

5.1 Outdoor dust loadings

Anspaugh (1974) and Anspaugh et al. (1975) proposed the use of a mass-loading model to 

address the resuspension of material from a stabilized source of radioactive materials within a 

soil matrix. For this model, it was assumed that the concentration of a radionuclide in an 

atmospheric aerosol would be the same as the concentration of that radionuclide in soil. Thus, 

the additional parameter of mass loading in air is required for the model. Anspaugh (1974) 

examined data from the National Air Surveillance Network for the year 1966 (NAPCA, 1968); 

217 urban and 30 non-urban stations were reporting data. The annual arithmetic mean for the 

urban stations ranged from 33 μg/m3

 (St. Petersburg, FL) to 254 μg/m3 (Steubenville, OH), with 

a mean for all 217 stations of 102 μg/m3. For the non-urban stations, the range was from 9 μg/m3

(White Pine County, NV) to 79 μg/m3 (Curry County, OR), with a mean for all 30 stations of 

38 μg/m3. Considering the range of the data, Anspaugh et al. (1975) proposed that a value of 

100 μg/m3 be used for a predictive value and presented several examples of data where that 

value appeared to be reasonable. It is important to note that measurements of mass loadings 

being made in 1966 were not based on specific particle sizes other than the standard housing in 

use, which was thought to sample all particles of aerodynamic diameter less than 

100 micrometers (μm). 
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Section 3.6 of Yu et al. (2015) is a relatively recent compendium of literature on outdoor dust 

loading by Argonne National Laboratory. The emphasis is on the average outdoor dust loading, 

but also includes references to reports addressing outdoor dust loading associated with 

excavation activities. According to Section 3.6.2 (pp. 111–112) of that report: 

The 24-hr weighted air concentrations of PM-2.5 particles for FY 2011–2014 

from the EPA’s Air Data web site were reviewed for more than 1000 counties in 

the U.S. The ambient average PM-2.5 air concentration ranged from 1 × 10-6 to 

4.5 × 10-5 g/m3, and the 98th percentile PM-2.5 air concentrations ranged from

5 × 10-6 to 1.7 × 10-4 g/m3 . . . . Healy and Rodgers (1979) used 1 × 10-4 g/m3 for 

predictive purposes and found that the predicted results and the real cases were 

comparable. The value of mass loading, 1 × 10-4 g/m3, used for predictive 

purposes is more conservative compared to the average ambient concentrations 

observed and takes into account the enhanced activity concentration in 

resuspended particulates compared to the activity concentration in soil. The EPA 

(EPA 1977) has used the same value to screen calculations. 

The mass loading value will fluctuate above its ambient level depending on 

human activities such as plowing and cultivating dry soil or driving on an 

unpaved road. The estimated mass loading for construction activities is about 

6.0 × 10-4 g/m3

; for exposure to construction traffic on unpaved roads, it is 

4.0 × 10-4 g/m3

; and for agriculture-generated dust, it is about 3.0 × 10-4 g/m3

([NRC] 1981). The maximum respirable dust loading inside the cab of heavy 

construction equipment during a surface coal mining operation was found to be 

1.8 × 10-3 g/m3 ([NRC] 1981). Estimates of mass loadings have been as high as 

1.3 g/m3 for instantaneous mass loadings during tilling. 

The U.S. Environmental Protection Agency (EPA) data mentioned in the first paragraph of this 

quoted passage are based on criteria related to specific particle sizes—in this case, only mass 

associated with particles with aerodynamic diameters of <2.5 μm. This reflects concern that 

particles of this size may represent health impacts due to the particles alone and not associated 

with any radioactivity. Radionuclides can be attached to particles much larger than 2.5 μm; it is 

generally considered that particles up to 100 μm or even larger can be inhaled. SC&A examined 

data from the same EPA source (http://www.epa.gov/airdata); tables 2–5 summarize these data

for 2011 to 2020. The data for particulate matter <10 μm (PM 10) are more relevant for our 

considerations. Values for PM 10 are about twice those for PM <2.5 μm (PM 2.5), but single 

maximum values as high as 3,400 μg/m3 are noted. Data reported in tables 2–5 are for the 

number of counties reporting as indicated under “Count.” All values are in μg/m3. Mono County, 

CA, and Pinal County, AZ, compete for the highest values of PM 10. 
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Table 2. 98th percentile values (PM 2.5) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020

Year Mean Count Max Min

2011 25 459 76 8

2012 23 457 160 7

2013 23 488 97 8

2014 22 484 82 7

2015 23 535 110 7

2016 21 327 150 8

2017 24 328 150 9

2018 25 333 170 8

2019 20 328 71 5

2020 29 330 520 7

Table 3. Weighted average values (PM 2.5) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020.

Year Mean Count Max Min

2011 9.8 459 20 3.4

2012 9.3 457 22 1

2013 9.0 488 23 3.1

2014 8.7 484 19 1.6

2015 8.5 535 21 2.2

2016 7.9 327 16 2.3

2017 8.3 328 18 2.6

2018 8.3 333 19 2.5

2019 7.8 328 16 0.9

2020 8.4 330 23 2.3

Table 4. Second maximum values (PM 10) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020

Year Mean Count Max Min

2011 82 320 3400 20

2012 81 327 2200 17

2013 72 315 1900 17

2014 76 301 1900 13

2015 74 288 980 18

2016 72 200 460 20

2017 87 195 1000 18

2018 90 193 830 20

2019 74 191 530 14

2020 110 192 620 16
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Table 5. Mean 24-hour values (PM 10) of mass loading extracted from 

http://www.epa.gov/airdata for 2011 through 2020

Year Mean Count Max Min

2011 21 320 140 7

2012 21 327 100 6

2013 20 315 96 6

2014 20 301 98 6

2015 20 288 65 6

2016 21 200 83 7

2017 21 195 87 6

2018 22 193 79 6

2019 19 191 59 5

2020 23 192 88 5

With respect to M&C, and excavation in general, the Yu et al. (2015) estimated mass loading for 

construction activities, 6.0 × 10-4 g/m3

 (600 μg/m3), appears somewhat applicable to the scenario 

of concern to this report, considering factors such as soil moisture and time averaging over the 

duration of the soil disturbance activities. Although Yu et al. (2015) do not specifically state it, 

SC&A presumes that this value of 600 μg/m3 is not limited to particles of size <2.5 μm.

NUREG/CR-1759, volume 3 (U.S. Nuclear Regulatory Commission (NRC), 1981, pp. A-5–

A-9), estimates a mass loading of 0.565 milligram per cubic meter (mg/m3) (565 μg/m3) during 

construction-related excavation activities. It also identifies 1980 experimental findings of 

“respirable dust loading ranging from 0.56 mg/m3 (for a bulldozer) to 6.7 mg/m3 (for a front end 

loader) within a few feet of the equipment” (NRC, 1981, pp. A-8–A-9). However, the 

characteristics of the soil (dry vs. wet) between scenarios might differ, as no information on the 

soil makeup is provided. Notwithstanding these issues, SC&A believes the dust loading values 

reported in NUREG/CR-1759, volume 3, tend to support the 212 μg/m3 NIOSH is using for the 

outdoor excavation scenario, but the NIOSH value does not appear to be necessarily bounding. 

The fact that the M&C soil was likely moist tends to provide a degree of assurance that the 

212 μg/m3 is reasonable. 

Vehicles on unpaved roads are recognized as large dust generators. At Mound, this was mitigated 

by wetting the unpaved roads. It is not known the degree to which this technique was used at 

M&C, though SC&A believes it is reasonable to assume there was some dust generated from 

vehicles on unpaved roads at M&C. NUREG/CR-1759, volume 3 (NRC, 1981, pp. A-10 and 

A-12), estimates a mass loading factor of 0.334 g/m3

 (334,000 ug/m3) from vehicular traffic on 

unpaved road and states: 

This value is considerably more than the value of 0.565 mg/ m3 calculated for the 

construction case. A meaningful average may be obtained from this value, 

however, by assuming that exposure of the individual to this peak concentration 

lasts about 30 seconds. It is unreasonable to assume that the individual would 

remain in the vehicular dust cloud for more than a few seconds. Further assuming 

that during a period of 500 hours (comparable to the construction duration) he is 

exposed to the maximum concentration of dust from 70 vehicles (about one 
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vehicle per working day), yields an average exposure mass loading of about 

0.390 mg/m3

. 

This illustrates an important consideration when evaluating dust loading measurements with 

significant variation based on the work activity: the time period used for averaging (occupancy 

time). Dust loadings have a wide range of values from a single exposure event. While the peak 

dust loading may be high and could be used to bound exposures, it is unreasonable to assume all 

exposures occurred at that level. The time period assumed in averaging has a profound impact on 

the dust loading assumed. In this example, averaging the peak mass loading over a more typical 

exposure period reduces the mass loading average by orders of magnitude. 

NUREG/CR-1759, volume 3, also explores dust loadings during tillage activities (which may 

have some applicability to the M&C outdoor excavation scenario) as follows (NRC, 1981, 

p. A-14): 

The respirable particulate concentration calculated for tillage is about 6.7 times 

that for construction. Assuming that a construction event takes three months 

(about 500 working hours) and an agricultural season involves 3 soil tilling events 

(13.5 hours per tilling of the 100 acre site), exposure to construction-generated 

dust would be 12.35 times the duration of exposure to tillage dust. Averaging the 

agriculture-generated dust loading over 500 hours yields an airborne 

concentration of 0.304 mg/m3, which corresponds closely to the 500 hour 

construction scenario average of 0.565 mg/m3

, and is smaller than that associated 

with the unpaved road scenario. 

This further illustrates the effect that occupancy (i.e., scenario-specific) time averaging has on 

the evaluation of dust loading over a wide range of work activities. Perhaps more important to 

note, the 0.304 mg/m3

 (304 μg/m3) dust loading value compares reasonably well to the 

212 μg/m3

 value under consideration by NIOSH. 

Section 6.4.4 of NUREG/CR-5512, volume 3 (NRC, 1999), discusses and characterizes the 

enormous variability in outdoor dust loadings. For example, section 6.4.4.1 (p. 6-68) states the 

following about outdoor and indoor dust loading factors: 

The dust-loading factors are used to calculate the average annual dose resulting 

from inhalation of airborne contaminants. The dust-loading factors, CDO and 

CDG, are used to calculate the inhalation dose due to activities occurring 

outdoors. CDO (g/m3

) represents the mass concentration of contaminated airborne 

particles in air outdoors, as defined in the exposure model, and corresponds to the 

long-term average quantity of respirable particulate material in outdoor air. CDG 

(g/m3) represents the higher average mass loading of contaminated airborne 

particles in air while the individual is gardening. The default values for these 

parameters defined in Volume 1, are 1 × 10-4 g/m3

 for CDO and 5 × 10-4 g/m3 for 

CDG. These values were defined based on the review of literature from outdoor 

air pollution studies from the National Air Sampling Network and studies on 

suspended particles in the atmosphere in communities across the United States.
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SC&A believes that the gardening scenario, with a dust loading of 5E-4 g/m3 (500 μg/m3), is 

most applicable to the outdoor excavation scenario of concern for M&C. It is noteworthy that the 

dust loading associated with outdoor gardening is likely comparable to the type of exposures 

M&C workers might have experienced when up close to underground piping and conduits in 

both the indoor and outdoor settings. The 500 μg/m3 value is surprisingly close to the value 

given in NUREG/CR-1759, volume 3 (NRC, 1981), and has the same type of limitations when 

applied to M&C. In SC&A’s opinion, it tends to support the 212 μg/m3

 value under 

consideration by NIOSH as a reasonable but not necessarily bounding dust loading value on its 

own. However, other factors, such as occupancy time and contamination levels, impact the 

calculation of the intake value when evaluating whether it is a bounding exposure scenario 

overall. 

Section 6.4.4.3 of NUREG/CR-5512, volume 3 (NRC, 1999, p. 6-69), also states the following:

The outdoor air dust-loading factors range from 1 × 10-5 to 2.3 × 10-1 g/m3 for all 

airborne particles. Under extreme conditions, air dust-loading can be as high as 

5 g/m3; however, these conditions persist for only very short periods of time. For 

particles less than 10 μm diameter (the respirable fraction), air dust-loading 

factors range from 1 × 10-5 to 7 × 10-4 g/m3

. 

The range of values for respirable particles2

 of 1E-5 to 7E-4 g/m3 (10 to 700 μg/m3) bracket 

NIOSH’s dust loading range of 212 μg/m3. 

2 The respirable particulates are the airborne particulates that can penetrate beyond the bronchioles into the gas 

exchange regions of the lungs. 

5.2 Indoor dust loadings

Under most circumstances, indoor dust loadings associated with suspension processes from 

contaminated surfaces are calculated through the use of a resuspension factor, expressed in units 

of picocuries per cubic meter (pCi/m3) per picocuries per square meter (pCi/m2), which reduces 

to units of m-1. Resuspension factors are widely employed by NIOSH in dose reconstructions, 

site profiles, and SEC petition reviews. In addition, generic guidance addressing resuspension 

factors is provided in Battelle-TBD-6000 and OTIB-0070. However, M&C maintenance 

activities at times involved indoor excavation. In the case of excavation, the dust loading 

approach is generally more appropriate.

For M&C, NIOSH plans to use the same dust loading of 212 μg/m3, based on outdoor dust 

loading data associated with excavations from Mound (NIOSH, 2020). It is difficult to judge 

whether outdoor dust loading data associated with excavations can be used as a surrogate for 

indoor excavation activities. SC&A found there is limited research on dust loadings comparable 

to the indoor digging done at M&C where indoor soil is aggressively disturbed. However, 

NUREG/CR-5512, volume 1 (NRC, 1992, p. 6.11), states the following:

For this analysis, the radioactive concentrations in indoor air for the building 

renovation and residential scenarios have been assumed to be 10-4 and 

5 x 10-5 g/m3, respectively. This range is a fraction of the maximum total dust 
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limits, representing longer-term average concentrations and accounting for 

airborne dust from nonradioactive sources. This range provides a prudently 

conservative estimate of actual radioactive dust-loadings in the workplace or 

household, and serves as an adequate basis for the first-level generic screening 

analysis.

The renovation indoor dust loading of 10-4 g/m3 (100 μg/m3) might include excavation and 

would seem to independently support NIOSH’s use of 212 μg/m3 as a reasonable estimate when 

applied to the M&C subsurface indoor excavation scenario and also to indoor excavation dust 

loading in general. Since this is an indoor dust loading value, SC&A suggests that NIOSH also 

refer to NUREG/CR-5512 (in addition to the Mound outdoor data) as a basis for the selected dust 

loading for both M&C and for use in OTIB-0070. SC&A also suggests that NIOSH address 

enrichment/enhancement issues, as discussed in section 6 of this report, for outdoor dust 

loadings. 

6 Enrichment/Enhancement

The literature review in section 5 makes a distinction between total dust loading and dust loading 

of respirable particles. This distinction can be important because of a phenomenon referred to as 

enrichment or enhancement: not enrichment in terms of enriched uranium, but the enrichment or 

enhancement that often occurs when contaminants in soil or residue are suspended and tend to be 

enriched in smaller sized particles as compared to the soil from which the airborne dust 

originates.

Often, dust load factors that are used to derive inhalation doses from suspension processes are 

premised on the assumption that the concentration of a given residual radionuclide in soil or 

residue (for example, in units of picocuries per gram (pCi/g) dry weight) is the same as the 

concentration of a given radionuclide in suspended airborne dust (pCi/g dry weight). Tamura 

(1975) observed that plutonium in soil was preferentially attached to small to mid-size particles, 

which contained the majority of surface area. There is also evidence that the dust that is 

suspended is of a different particle size distribution than the soil from which it originates. The 

implications of this phenomenon are that the concentrations of many radionuclides in suspended 

dust are likely to be somewhat higher than in the soil or residue from which it originates. For 

example, Envirosphere (1984) cites Adams et al. (1980) enrichment factors of 1.5 for uranium 

and 3.2 for thorium. Enrichment/enhancement should be taken into consideration when the 

starting point for deriving inhalation doses is the concentration of a given radionuclide in soil 

followed by suspension. 

The relevant equation for the concentration of radioactive materials in air via the pathway of 

resuspension is as follows:

soil

air

soil

ML C EF

C EF

C

χ =× ×

= 
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where

χ = Concentration of radionuclide in air (Ci/m3

);

ML = Mass loading in air (g/m3);

Csoil = Concentration of radionuclide in soil (Ci/g);

EF = Enhancement factor

Cair = Concentration of radionuclide in airborne mass (Ci/g).

An analysis of enhancement factors was included in DOE (2006). These data are summarized in 

figure 3. There is a substantial difference in the values of enhancement factors depending upon 

whether soil is disturbed or not. For measurements of undisturbed soil, the geometric mean is 

0.59 with a geometric standard deviation of 2.0; for disturbed soil the geometric mean is 2.9 with 

a geometric standard deviation of 1.8. NIOSH should consider incorporating 

enrichment/enhancement factors into the calculation of inhalation doses associated with 

suspension processes outdoors.
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Figure 3. Plot of enhancement factors measured over undisturbed and disturbed soils

Source: Data are presented in DOE (2006). 

7 Conclusions

Considering the totality of information compiled in this report, SC&A believes that the use of a 

dust loading of 212 μg/m3 for subsurface work both indoors and outdoors at M&C is reasonably 

compatible with data and information summarized in this report, including the data reported from 

Mound by the interviewed SME. SC&A concludes that NIOSH’s adoption of 212 μg/m3 for 

estimating respirable outdoor dust loading during excavation activities is reasonable but not 

necessarily bounding. Additionally, SC&A believes that NIOSH should refer to the numerous 
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dust loading studies cited in section 5 as the basis for the dust loading of 212 μg/m3 in addition to 

the Mound data. 

While SC&A’s survey and interpretation of the data indicate that the suggested value of 

212 μg/m3 may not necessarily be sufficiently conservative for many excavation scenarios, a 

number of mitigating factors are also present at M&C that should be considered. First, the soil at

M&C was likely moist. Second, the dust loading used for dose reconstruction at M&C covered 

the entire assumed time period of 2 months; i.e., the suggested exposure models are not trying to 

reconstruct short-term exposures, where dust loading might peak during active and aggressive 

excavation, but are making use of the dust loading for deriving inhalation exposures over a more 

protracted period of time.

With respect to using 212 μg/m3as a generic value in OTIB-0070 for dust loading associated 

with excavation at other Energy Employees Occupational Illness Compensation Program Act

sites, SC&A believes that such a generic strategy may be problematic. As discussed in this

report, dust loading, especially dust loading associated with excavation, is extremely dependent 

on many site-specific factors. Hence, it would be difficult to pick a default dust loading that 

would be universally applicable. The best that can be said is that it is unlikely that workers would 

be exposed for extended periods of time to respirable dust loadings above a few mg/m3. It can 

also be stated with a degree of certainty that it would be highly implausible that workers could be

exposed for extended periods of time to dust loading of any size distribution of about 100 mg/m3

(100,000 μg/m3) or greater because of the choking effect of such high levels. As Stewart (1967) 

notes, “The high degree of stirrup of dust produced by the experiments resulted in high airborne

dust loadings 110 mg/m3is barely tolerable.” Finally, NIOSH should discuss the potential effect 

of enrichment factors on dust loading in both the specific context of the M&C methodology and 

how it may apply to other sites if the suggested dose loading of 212 μg/m3is to be used as a 

generic methodology in the OTIB-0070 guidance. 

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